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Temporal variations in ice sheet flow directly impact the internal structure within ice sheets through englacial deformation. Large-scale changes in the vertical stratigraphy within ice sheets have been previously conducted on centennial to millennial timescales; however, intra-annual changes in the morphology of internal layers have yet to be explored. Over a period of two years, we use autonomous phase-sensitive radio-echo sounding (ApRES) to track the daily displacement of internal layers on Store Glacier, West Greenland to millimeter accuracy. At a site located ~30 km from the calving terminus, where the ice is ~600m thick and flows at ~700m/yr, we measure distinct seasonal variations in vertical velocities and vertical strain rates over a two-year period. Prior to the melt season (March–June), we observe increasingly non-linear englacial deformation with negative vertical strain rates (i.e. strain thinning) in the upper half of the ice column of ~0.03 a-1, whereas the ice below thickens under vertical strain reaching up to 0.16 a-1. Early in the melt season (June–July), vertical thinning gradually ceases as the glacier increasingly thickens. During late summer to midwinter (August–February), vertical thickening occurs linearly throughout the entire ice column, with strain rates averaging 0.016 a-1. We show that these complex variations are unrelated to topographic setting and localized basal slip, and hypothesize that this seasonality is driven by far-field perturbations in the glacier’s force balance, in this case generated by variations in basal hydrology near the glacier’s terminus and propagated tens of kilometers upstream through longitudinal coupling.

This work is funded by the Netherland Polar Programme, Netherlands Earth System Science Centre (NESSC), NSF OPP research grant 0732946, NERC/GEF grants NE/L006707/1, NE/L005409/1, NE/E012914/1, GEF loans 863, 890, 1028. The surface mass balance (SMB) of the Larsen C ice shelf (LCIS), Antarctica, is poorly constrained due to a dearth of in situ observations. Combining several geophysical techniques, we reconstruct spatial and temporal patterns of SMB over the LCIS. Continuous time series of snow height (2.5–6 years) at five locations allow for multi-year estimates of seasonal and annual SMB over the LCIS. There is high interannual variability in SMB as well as spatial variability: in the north, SMB is 0.40 ± 0.06 to 0.41 ± 0.04 m w.e. year−1, while farther south, SMB is up to 0.50 ± 0.05 m w.e. year−1. This difference between north and south is corroborated by winter snow accumulation derived from an airborne radar survey from 2009, which showed an average snow thickness of 0.34 m w.e. north of 66° S, and 0.40 m w.e. south of 68° S. Analysis of ground-penetrating radar from several field campaigns allows for a longer-term perspective of spatial variations in SMB: a particularly strong and coherent reflection horizon below 25–44 m of water-equivalent ice and firn is observed in radargrams collected across the shelf. We propose that this horizon was formed synchronously across the ice shelf. Combining snow height observations, ground and airborne radar, and SMB output from a regional climate model yields a gridded estimate of SMB over the LCIS. It confirms that SMB increases from north to south, overprinted by a gradient of increasing SMB to the west, modulated in the west by föhn-induced sublimation. Previous observations show a strong decrease in firn air content toward the west, which we attribute to spatial patterns of melt, refreezing, and densification rather than SMB. Publisher PDF

Surface melting has been contributing to the surface lowering and loss of firn air content on Larsen C Ice Shelf since at least the mid-1990s. Where the amount of melting and refreezing is significant, the firn can become impermeable and begin to support ponds of surface meltwater such as have been implicated in ice shelf collapse. Although meteorological station data indicated an increase in melt on the Antarctic Peninsula over the second half of the 20th century, the existing Ku-band Quick Scatterometer (QuikSCAT) time series is too short (1999–2009) to detect any significant 21st century trends. Here we investigate a longer 21st century period by extending the time series to 2017 using the C-band Advanced Scatterometer (ASCAT). We validate our recent observations with in situ weather station data and, using a firn percolation model, explore the sensitivity of scatterometry to water at varying depths in the firn. We find that active microwave C-band (5.6-cm wavelength) instruments can detect water at depths of up to 0.75 m below a frozen firn layer. Our longer scatterometry time series reveals that Larsen C Ice Shelf has experienced a decrease in melt season length of 1–2 days per year over the past 18 years consistent with decreasing summer air temperatures. Only in western inlets, where föhn winds drive melt, has the annual melt duration increased during this period.

This work was funded by the Conoco Phillips-Lundin Northern Area Program through the CRIOS project (Calving Rates and Impact On Sea level, http://www.researchinsvalbard.no/project/7037). Penelope How is supported by a NERC PhD studentship. Subglacial hydrological processes at tidewater glaciers remain poorly understood due to the difficulty in obtaining direct measurements and lack of empirical verification for modelling approaches. Here, we investigate the subglacial hydrology of Kronebreen, a fast-flowing tidewater glacier in Svalbard during the 2014 melt season. We combine observations of borehole water pressure, supraglacial lake drainage, surface velocities and plume activity with modelled run-off and water routing to develop a conceptual model that thoroughly encapsulates subglacial drainage at a tidewater glacier. Simultaneous measurements suggest that an early-season episode of subglacial flushing took place during our observation period, and a stable efficient drainage system effectively transported subglacial water through the northern region of the glacier tongue. Drainage pathways through the central and southern regions of the glacier tongue were disrupted throughout the following melt season. Periodic plume activity at the terminus appears to be a signal for modulated subglacial pulsing, i.e. an internally driven storage and release of subglacial meltwater that operates independently of marine influences. This storage is a key control on ice flow in the 2014 melt season. Evidence from this work and previous studies strongly suggests that long-term changes in ice flow at Kronebreen are controlled by the location of efficient/inefficient drainage and the position of regions where water is stored and released. Publisher PDF

We use borehole optical televiewing (OPTV) to explore the internal structure of Larsen C Ice Shelf (LCIS). We report a suite of five ~90 m long OPTV logs, recording a light‐emitting diode‐illuminated, geometrically correct image of the borehole wall, from the northern and central sectors of LCIS collected during austral spring 2014 and 2015. We use a thresholding‐based technique to estimate the refrozen ice content of the ice column and exploit a recently calibrated density‐luminosity relationship to reveal its structure. All sites are dense and strongly influenced by surface melt, with frequent refrozen ice layers and mean densities, between the depths of 1.87 and 90 m, ranging from 862 to 894 kg m−3. We define four distinct units that comprise LCIS and relate these to ice provenance, dynamic history, and past melt events. These units are in situ meteoric ice with infiltration ice (U1), meteoric ice which has undergone enhanced densification (U2), thick refrozen ice (U3), and advected continental ice (U4). We show that the OPTV‐derived pattern of firn air content is consistent with previous estimates, but that a significant proportion of firn air is contained within U4, which we interpret to have been deposited inland of the grounding line. The structure of LCIS is strongly influenced by the E‐W gradient in föhn‐driven melting, with sites close to the Antarctic Peninsula being predominantly composed of refrozen ice. Melting is also substantial toward the ice shelf center with >40% of the overall imaged ice column being composed of refrozen ice.

Research was funded by the UK Natural Environmental Research Council grants NE/L006707/1 and NE/L005409/1 and a HEFCW/Aberystwyth University Capital Equipment Grant to B.H. Data will be available via the project website (www.projectmidas.org) and the UK Polar Data Centre (https://www.bas.ac.uk/data/uk-pdc/) from mid-2017. We use borehole optical televiewing (OPTV) to explore the internal structure of Larsen C Ice Shelf (LCIS). We report a suite of five ~90 m long OPTV logs, recording a light-emitting diode-illuminated, geometrically correct image of the borehole wall, from the northern and central sectors of LCIS collected during austral spring 2014 and 2015. We use a thresholding-based technique to estimate the refrozen ice content of the ice column and exploit a recently calibrated density-luminosity relationship to reveal its structure. All sites are dense and strongly influenced by surface melt, with frequent refrozen ice layers and mean densities, between the depths of 1.87 and 90 m, ranging from 862 to 894 kg m−3. We define four distinct units that comprise LCIS and relate these to ice provenance, dynamic history, and past melt events. These units are in situ meteoric ice with infiltration ice (U1), meteoric ice which has undergone enhanced densification (U2), thick refrozen ice (U3), and advected continental ice (U4). We show that the OPTV-derived pattern of firn air content is consistent with previous estimates, but that a significant proportion of firn air is contained within U4, which we interpret to have been deposited inland of the grounding line. The structure of LCIS is strongly influenced by the E-W gradient in föhn-driven melting, with sites close to the Antarctic Peninsula being predominantly composed of refrozen ice. Melting is also substantial toward the ice shelf center with >40% of the overall imaged ice column being composed of refrozen ice. Publisher PDF

Following a southward progression of ice-shelf disintegration along the Antarctic Peninsula (AP), Larsen C Ice Shelf (LCIS) has become the focus of ongoing investigation regarding its future stability. The ice shelf experiences surface melt and commonly features surface meltwater ponds. Here, we use a flow-line model and a firn density model (FDM) to date and interpret observations of melt-affected ice layers found within five 90m boreholes distributed across the ice shelf. We find that units of ice within the boreholes, which have densities exceeding those expected under normal dry compaction metamorphism, correspond to two climatic warm periods within the last 300 years on the Antarctic Peninsula. The more recent warm period, from the 1960s onwards, has generated distinct sections of dense ice measured in two boreholes in Cabinet Inlet, which is close to the Antarctic Peninsula mountains – a region affected by föhn winds. Previous work has classified these layers as refrozen pond ice, requiring large quantities of mobile liquid water to form. Our flow-line model shows that, whilst preconditioning of the snow began in the late 1960s, it was probably not until the early 1990s that the modern period of ponding began. The earlier warm period occurred during the 18th century and resulted in two additional sections of anomalously dense ice deep within the boreholes. The first, at 61m in one of our Cabinet Inlet boreholes, consists of ice characteristic of refrozen ponds and must have formed in an area currently featuring ponding. The second, at 69m in a mid-shelf borehole, formed at the same time on the edge of the pond area. Further south, the boreholes sample ice that is of an equivalent age but which does not exhibit the same degree of melt influence. This west–east and north–south gradient in the past melt distribution resembles current spatial patterns of surface melt intensity.

Surface melt ponds form intermittently on several Antarctic ice shelves. Although implicatedin ice-shelf break up, the consequences of such ponding for ice formation and ice-shelfstructure have not been evaluated. Here we report the discovery of a massive subsurface icelayer, at least 16 km across, several kilometres long and tens of metres deep, located in anarea of intense melting and intermittent ponding on Larsen C Ice Shelf, Antarctica. Wecombine borehole optical televiewer logging and radar measurements with remote sensingand firn modelling to investigate the layer, found to be ~10°C warmer and ~170 kg/m³ denser than anticipated in the absence of ponding and hitherto used in models of ice-shelffracture and flow. Surface ponding and ice layers such as the one we report are likely to formon a wider range of Antarctic ice shelves in response to climatic warming in forthcomingdecades., 0, SCOPUS: ar.j, info:eu-repo/semantics/published

Surface melt ponds now form frequently on ice shelves across the northern sector of the Antarctic Peninsula in response to regional warming and local föhn winds. A potentially important, but hitherto unknown, consequence of this surface melting and ponding is the formation of high-density near-surface ice from the refreezing of that water. We report the discovery and physical character of a massive subsurface ice layer located in an area of intense melting and intermittent ponding on Larsen C Ice Shelf, Antarctica. We combine borehole optical televiewer logging and ground-based radar measurements with remote sensing and firn modelling to investigate the formation and spatial extent of this layer, found to be tens of kilometres across and tens of metres deep. The presence of this ice layer has the effect of raising local ice shelf density by about 190 kgm-3 and temperature by 5 - 10 degrees C above values found in areas unaffected by ponding and hitherto used in models of ice-shelf fracture and flow.